These three knowledge topics for this objective overlap each other so I “condensed” them into single section.– Identify available Storage Load Balancing options– Identify available Storage Multi-pathing Policies– Identify features of Pluggable Storage Architecture (PSA)
Managing Multiple Paths in the vSphere Storage Guide on page 184.

The Pluggable Storage Architecture (PSA) is a collection of Storage APIs.
The PSA allows 3rd party software developers to design their own load balancing techniques and failover mechanisms for particular storage array, and insert their code directly into the ESXi storage I/O path.

SATPs and PSP can be built in (Native) or can be provided by a 3rd party. Multiple third-party MPPs can run in parallel with the VMware NMP. Third-party MPPs replace the behavior of the NMP and take complete control of the path failover and the load-balancing operations for specified storage devices.

The SATP monitors the health of the physical paths to storage, reports changes in path state, and handles path failover for a given storage array.

The PSP handles the path selection for a given device.
Default NMP PSPs:

VMW_PSP_MRU – Most Recently Used (MRU)The host selects the path used most recently. If that path becomes unavailable the host selects another path, the host will not revert back to the original path if it becomes available again. MRU is the default policy for most active-passive storage devices.

VMW_PSP_FIXED – FixedA uses a designated preferred path or the first working path if a preferred path has not be set. Fixed is the default policy for most active-active storage devices.

VMW_PSP_RR – Round RobinThe host rotates through all active paths when connecting to active-active arrays or through all available paths when connecting to active-active arrays.

When using Fixed path policy the preferred path is marked with an asterisk (*) in the preferred column.

The Round Robin PSP defaults to an IOPS limit of 1000 per path. 1000 I/O are sent down each path before switching to the next path. This can be adjusted with esxcli. Alternatively a bytes limit can be set on the paths when using RR.

If no SATP is assigned to the device by the claim rules, the default SATP for iSCSI or FC devices is
VMW_SATP_DEFAULT_AA. The default PSP is VMW_PSP_FIXED.

Path status:

Active – Path is available for IO. Paths currently used for IO are marked as Active (I/O)

Standby – Path can become available if an active path fails.

Disabled – Path has been disabled and cannot process IO.

Dead – Unable to connect to storage through this path.

Managing Storage Paths and Multipathing Plug-Ins in the vSphere Storage Guide on page 192.
The esxcli is used to mange PSA multipathing plug-ins and the storage paths assigned to them.

You can group Virtual SAN hosts that could potentially fail together by creating a fault domain and assigning one or more hosts to it. Failure of all hosts within a single fault domain is treated as one failure. If fault domains are specified, Virtual SAN will never put more than one replica of the same object in the same fault domain.

Virtual SAN requires a minimum of 2*n + 1 fault domains in the cluster.

For example – hosts in one rack in one fault domain, hosts in another rack are in another, and hosts in another rack in yet another. VSAN policy to tolerate a single host failures will keep the data across fault domains. A fault domain appears as a single host to VSAN.

vHersey

Hersey Cartwright is an IT professional with extensive experience designing, implementing, managing, and supporting technologies that improve business processes. Hersey is Solutions Architect for HPE SimpliVity covering Virginia, Washington DC, and Maryland. He holds the VMware Certified Design Expert (VCDX-DV #128) certification. Hersey actively participates in the VMware community and was awarded the VMware vExpert title in 2016, 2015, 2014, 2013, and 2012. He enjoys working with, teaching, and writing about virtualization and other data center technologies. Follow Hersey on Twitter @herseyc